The ability to accurately control motion of a structure in three-dimensional space, or to control motion of one structure relative to another structure in a given space, poses problems of significant technological and economic consequence to many manufacturing applications, such as those used to fabricate semiconductor chips, printed circuit boards, liquid crystal displays, and thin film devices. These operations employ specialized structures (such as reticle and wafer stages in lithography machines, metrology stages, pick-and-place equipment, wafer-handling robots, gantry/head assemblies, linear motors, photoimaging systems, and etching systems) to manufacture and inspect these often-delicate and sensitive products.
A wafer stage supports a silicone wafer on which integrated circuits are printed at several locations called sie spots. A reticle stage supports a reticle (or mask) which is a master image of one layer of the integrated circuit that is to be created. A laser beam is used to illuminate the reticle. The illumination of the reticle results in an image that is directed on to the die spot of the silicon wafer. The wafer is coated with a photoresist that reacts with the illuminated image such that an image is created on the wafer. Subsequent processing of the wafer creates the circuitry (e.g. conductive traces) of the chip. Typically, a reticle contains a plurality of identical images. Multiple images are created on a wafer in a given processing cycle to create multiple chips on a single wafer. Precise control is required during these steps to ensure that each layer of the chip is accurately aligned relative to each other layer. The reticle and the wafer are each moved in opposite direction (scanned) during the illumination of each layer of each die image. A laser interferometer positioning system is often used to measure the position, velocity, and acceleration of the stage. In this machine after the illumination of a die layer (one layer of a chip) on a wafer, the wafer is moved (scanned) by the wafer stage in a particular direction and the reticle stage is moved in the opposite direction so that an adjacent die spot on the wafer can be illuminated. Thus, the process is simetimes referred to as a step and seam process. The process is repeated until all the spots on the wafer have been illuminated. Voice coil motors or linear magnetic actuators are often used to rapidly position the stages that support the semiconductor wafer and the reticle. For additional background information on these step and scan machines see Levinson, H. J.; Principles of Lithography, SPIE—The International Society for Optical Engineering, Bellingham Wash., 2001.
The image on the wafer is then utilized in the process of creating within a semiconductor device. These processes are generally repeated multiple times creating layers of the fine circuitry at each die spot on the wafer. Alignment of these layers can be critical to the performance of the devices. Alignment errors of several nm can sometimes be sufficient to render a device useless or severely limit its performance.
Chip-making processes have been speeded up through the use of advanced photolithography lasers such as those sold by Cymer, Inc. of San Diego, Calif., chip throughput requirements have also increased. One consequence of the increased requirements has been the need for much faster and more accurate positioning of photolithography stages. Faster positioning has created the need for more precise control of the movement and positioning of stages.
Active vibration and motion control provides one promising method of achieving adequate system governance. Active control is often an ideal technology for dealing with vibration and motion control issues for a number of reasons, such as those discussed in commonly-owned U.S. patent application Ser. Nos. 09/491,969 and 10/074,059, which are hereby incorporated by reference. However, unknowns in plant dynamics and unforeseen disturbances to the system being controlled can significantly alter the actual results attained through active structural control, especially when used with sensitive machines such as semiconductor capital equipment. In this context, disturbances can manifest themselves in a variety of ways, such as affecting the signals input to the system being controlled, causing variances in sensor signals or by impacting performance variables. In addition, uncertainty in base or stage dynamics, and the impact upon those dynamics caused by changes in equipment configuration, mass distribution, and aging of equipment, subsystems, or components, all may serve to limit the performance of any standard control method chosen.
Systems implemented in many manufacturing system to control the motion of various components of subsystems are often referred to as servo control systems. These systems incorporate various actuators and sensors to monitor and command a prescribed motion of the subsystem. In many such systems implementing high precision control there is a need to apply structural control in order to eliminate performance degrading vibration. The structural control system may incorporate additional actuators and sensors into the system and may use feedback control to damp out unwanted vibrations. These additional actuators and sensors add to the cost, design and operational complexity of manufacturing equipment.
What is needed is a better motion control system.